Photocatalytic intermolecular [2 + 2] cycloaddition/dearomatization of indoles: easy access to cyclobutane-fused indolines

Visible-light-promoted [2 + 2] cycloaddition/dearomatization of aromatic compounds is an efficient strategy for the synthesis of three-dimensional rigid four-membered rings to realize the modification of organic molecules, which boasts great potential and prospects in developing first-class drugs. Herein, cyclobutane-fused indolines were successfully synthesized via a photocatalytic intermolecular [2 + 2] cycloaddition/dearomatization reaction with olefins. This reaction exhibited excellent regio- and stereoselectivity (>20 : 1 dr) and high atom- and step-economy, and is characterized by mild and green reaction conditions, simple operation, and a wide range of substrate applicability. The mechanistic control exploration suggested that the [2 + 2] cycloaddition reaction was initiated by the triplet–triplet energy transfer paradigm.